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Abstract

A cold-rolled steel sheet of the present invention which has a composition containing, in terms of % by mass, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%), and Al: 0.001-0.10%, and remainder being mainly iron, and which has a structure comprising, in terms of area ratio, 10-80% ferrite, less than 5% (including 0%) of the sum of retained austenite and martensite, and a hard phase as the remainder. The steel sheet gives a KAM value frequency distribution curve in which the relationship between the proportion of frequency having a KAM value ≦0.4, X_KAM≦0.4°, and the area ratio of ferrite, V_α satisfies X_KAM≦0.4°/V_α≧0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, X_{KAM=0.6-0.8°} is 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 μm or larger exist so that three or less such cementite grains are dispersed per μm² of the hard phase. The steel sheet has improved balance between elongation and stretch flangeability and has better formability.

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First Claim